The present invention relates to a high definition color television transmission system in which a wide band television signal, having adjacent low and high frequency components and n lines per picture, is converted into a narrower band television signal having m lines per picture, where m is substantially 1/2n, with both signals having the same field and picture rates, said converted television signal having adjacent low and high frequency luminance portions, the low frequency luminance portion in respective adjacent lines of a field of said converted television signal being derived from the low frequency components in respective alternate lines in a field of said wide band signal. The invention also relates to transmission and reception equipment for use with such a system.
U.S. Pat. No. 3,830,971 describes a line standard converter in which a television signal having a first number of lines is converted into a second television signal with substantially half the number of lines. The system described therein only takes information from alternate lines of the first signal and thus, if the second signal was restored to a signal having the same number of lines as the first signal, then only half the information for this signal would be present such that this signal would be of poor definition when compared with the first signal from whence it originated.
The color television signal currently transmitted by the broadcast authorities in the United Kingdon uses the PAL system with the transmissions taking place in the U.H.F. bands IV and V. In such transmissions, the luminance information has a bandwidth of 5.5 MHz with a color subcarrier located at a point nominally 4.43 MHz above the carrier frequency, the color subcarrier being modulated by the color information. Substantially all of the currently manufactured color television receivers do not fully utilize all the luminance information transmitted. The luminance information above 3.5 MHz in a receiver is reduced in magnitude to simplify the operation of decoding the color information and, without such simplified decoding, interference between the luminance and color information signals would be observed on the display. Considerable interest has however been shown in increasing the quality of the display and this can be done by using the whole of the luminance information transmitted.
Various suggestions have been made to further increase the bandwidth and hence the quality and definition of transmitted color television signals. One such suggestion contemplates the transmission of a single sideband luminance modulated signal of 10 MHz bandwidth with the modulated color subcarrier being located in the other sideband. Such a transmission could not readily be currently transmitted in the U.H.F. bands IV or V as the required bandwidth would overlap into an adjacent channel. In addition, such a transmission could not be received by television receivers currently manufactured and would thus only be receivable by specially constructed receivers. A further suggestion proposed by NHK (Japan Broadcasting Corporation) in their NHK Laboratories Note, Ser. No. 239, August 1979 contemplates the separate transmission of luminance and colour information in separate channels and suffers from similar objections to those above. Yet a further suggestion comes from the BBC (British Broadcasting Corporation) in their Engineering Press Release "The BBC moves towards better quality pictures", dated Feb, 23, 1982 where, for wide band transmission such as those via a satellite or optical fiber cable, it is suggested that the high frequency luminance information above 3.5 MHz be filtered off and this higher frequency information shifted upwards to a higher band (8 MHz upwards) and transmitted together with the original low frequency information and chrominance signals. Such a system cannot be used with current channel spacing in the UHF bands IV and V and although the low frequency luminance information and chrominance signals could be used to provide a reasonable display in most currently manufactured color television receivers, any receiver designed to utilize the full 5.5 MHz luminance bandwidth would produce a display that is impaired by the signal of this suggestion.